Field of the Invention
This invention relates to bar code readers.
A bar code is a pattern of dark bars and light spaces between the bars, representing numbers and/or letters according to a specific rule. A bar code character is a group of bars and spaces that represent a single number or letter. A bar code symbol is a collection of several bar code characters which represent a required comprehensive identification.
Scanning bar code readers include a detector which produces a series of electronic signals as the symbol is scanned. A first type of reader is known in which the symbol is scanned by a spot of light and the detector senses the entire scene in which the symbol is located. The detector senses the modulation of the light reflected from the spot as the spot traverses the symbol. A second type of reader is one which not only scans the symbol with a spot of light, but scans an image of the detector along with the spot. Such an improved reader has the advantage that the detector only sees a relatively small area around and including the instantaneous position of the spot. A third type of reader is one in which the symbol is uniformly illuminated and an image of the detector is scanned across the symbol. In this third type of reader, the light received by the detector is modulated in accordance with the reflectivity of the bar or space on which the detector is instantaneously imaged.
The ability of a reader to read a symbol satisfactorily is dependent on its resolution; i.e., its ability to read the narrowest bar or space. In the first and second types of reader described above, resolution is dependent on the size and sharpness of the spot of light. In the second and third types of reader, resolution is dependent on the size and sharpness of the image of the detector on the symbol. If the diameter of the light spot or the width of the detector image is much greater than the width of the narrowest bar or space and/or the light spot or detector image is not sharply defined, then the modulation of the light incident on the detector as the light spot and/or detector image scans the narrowest bar or space may be inadequate to provide a signal from the detector which is adequately modulated for the interpretive electronics to which it is fed. Thus, two problems occur when endeavoring to increase the range over which a reader reads satisfactorily; one concerns focussing and the other concerns magnification.